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Humidity issue

Hello all! I have an issue, and I'm hoping someone can provide some insight as to what is wrong with my system...

First, here are the facts. I have a 3 zone system, with all 3 units being 20 seer American Standard Platinum ZM A/c units with matching variable speed air handlers. Two of the units are 2.5 ton units, and the other is a 4 ton unit for the main living space. In total, I have about 4500 sq ft of living space, with 40% of the home having vaulted ceilings (10+ feet).

Two of the zones are cooling fine, and maintain relative humidity levels around 40-45%. However, one of the units consistently runs at about 55-70% humidity, and that are of the house just feels wet. Everything was replaced when the new units were installed last year, including ducting, vents, returns, etc.

So, Since I've had the a/c company out twice, and they told me nothing was wrong, I bought a humidity gauge and started some testing of my own. On the two units that seem to be able to maintain a comfortable level of humidity, these are readings: supply temp of 10-16 degrees below return air temp and humidity level at supply of 30-40%. On the unit I feel is malfunctioning, I am getting a supply temp of 10-16 degrees below return air temp, but the humidity levels are 75-90%.

Any suggestions on what is going on, and what I can tell my a/c company to look at? Or, is there anything else I can test to narrow the scope of the issue???

^Basics are fan in "auto" mode to avoid rapid re-evaporation during short cooling cycles. A/cs with warm cooling coils do not remove enough moistue. For <50%RH, the cooling coil should be <50^F. The cooling system must be properly charged and correctly adjusted air flow. If the a/c is short cycling, increase the dead band of the t-Stat. As the amount of cooling decreases during the sholder seasons, supplemental dehumidification will be needed to maintain <50%RH during cool wet weather. Whole house dehumidifiers are devices that will maintain <50%RH without any cooling load at a very minimum energy cost.
Regards TB

Hello all! I have an issue, and I'm hoping someone can provide some insight as to what is wrong with my system...

First, here are the facts. I have a 3 zone system, with all 3 units being 20 seer American Standard Platinum ZM A/c units with matching variable speed air handlers. Two of the units are 2.5 ton units, and the other is a 4 ton unit for the main living space. In total, I have about 4500 sq ft of living space, with 40% of the home having vaulted ceilings (10+ feet).

Two of the zones are cooling fine, and maintain relative humidity levels around 40-45%. However, one of the units consistently runs at about 55-70% humidity, and that are of the house just feels wet. Everything was replaced when the new units were installed last year, including ducting, vents, returns, etc.

So, Since I've had the a/c company out twice, and they told me nothing was wrong, I bought a humidity gauge and started some testing of my own. On the two units that seem to be able to maintain a comfortable level of humidity, these are readings: supply temp of 10-16 degrees below return air temp and humidity level at supply of 30-40%. On the unit I feel is malfunctioning, I am getting a supply temp of 10-16 degrees below return air temp, but the humidity levels are 75-90%.

Any suggestions on what is going on, and what I can tell my a/c company to look at? Or, is there anything else I can test to narrow the scope of the issue??? Thanks in advance for your help!

You have 9-Tons of cooling for 4500-sf; that's the old 500-sf per ton equation that is usually way oversized. The new tonnage varies depending on the home's efficiency level. Usually it will be around 700 to 750-sf per ton, more or less.

Many homes can be weatherized to the 700-sf or, considerable more per ton level - depending on the climate conditions. This results in the needed longer runtime cycles for improved dehumidification.

The evaporator coil's temperature has to be well below the dew point temperature of the air moving through it; Tech may need to lower blower CFM level; especially on ultra high 20-SEER units I'd run the airflow at 350-CFM per ton of cooling or slightly less.

Depending on the structure of the home & how the home's supply air & return air is ducted, the warmer air at ten foot ceiling level can act as an insulator from the hot attic air.

How many sf does each unit cover &, was a room by room heatload calc ever performed?

Find out if any utility company will do a free or low cost Energy Efficiency Audit on your home.

Thank you all for the feedback. Let me add/correct some of the info I provided with more detail and answer some of your questions.

I misspoke, as the two smaller units are 2 ton units, not 2.5 ton units. Also, the home is 4763 sq ft, to be exact. I usally use 4500 as a rounded figure just to make it easy.

The unit in question is cooling approximately 1200 sq ft, almost the exact same as the other 2 ton unit. The only difference is that the unit in question is cooling an area with vaulted ceilings, as the other 2 ton unit is cooling bedrooms/bathrooms with the normal 8 ft ceilings.

I even gone to the extent of measuring the temp/humidity on both stage 1 and stage 2 cooling, and both have elevated humidity levels at the supply's compared to the same settings on the other two units. Also, the air handler is set to factory defaults, just like the other two units. So, everything is identical on all 3 units, except this one blows air at twice the humidity???

I've checked the ducting for leaks, and there doesn't seem to be any. Also, all of the windows and sliders in the home were replaced 6 months ago with dual paned impact glass, and they are very well sealed. So, we've been able to rule out air leaks bringing in the humid air from outside.

I guess my question is this... If the unit is set up correctly (fan speed), why would it still be cooling the air, but not removing any humidity?

q[QUOTE=falkirk18;13923381
The unit in question is cooling approximately 1200 sq ft, almost the exact same as the other 2 ton unit. The only difference is that the unit in question is cooling an area with vaulted ceilings, as the other 2 ton unit is cooling bedrooms/bathrooms with the normal 8 ft ceilings.

I even gone to the extent of measuring the temp/humidity on both stage 1 and stage 2 cooling, and both have elevated humidity levels at the supply's compared to the same settings on the other two units. Also, the air handler is set to factory defaults, just like the other two units. So, everything is identical on all 3 units, except this one blows air at twice the humidity???

I've checked the ducting for leaks, and there doesn't seem to be any. Also, all of the windows and sliders in the home were replaced 6 months ago with dual paned impact glass, and they are very well sealed. So, we've been able to rule out air leaks bringing in the humid air from outside.

I guess my question is this... If the unit is set up correctly (fan speed), why would it still be cooling the air, but not removing any humidity?[/QUOTE]

Time for a tech to measure the coil temp of the a/c in question. You must be able to take temp/%RH and convert to the dew point of the air. The %RH does not provide the dew point. Typical temps/%RH of a correctly setup a/c would be like 55^F, 80%RH equals 48^F dew point. Looks like you are oversized with a/c. You get all of your a/c coils <50%RH and minimize the number of short cycles as much as possible. Then add a 100 pint dehumidifier to the dampest area. This will get you to <50%RH throughout the home. Perfectly sized a/c will get you <50%RH during high cooling loads. You will still have high %RH during low/no cooling loads because the a/cs do not have enough cooling. For perfect humidity control, supplemental dehumidification is needed when the outdoor humiditiy is above the desired indoor humidity. Check out the Ultra-Aire 105 H whole house dehumidifier.

Thank you all for the feedback. Let me add/correct some of the info I provided with more detail and answer some of your questions.

I misspoke, as the two smaller units are 2 ton units, not 2.5 ton units. Also, the home is 4763 sq ft, to be exact. I usally use 4500 as a rounded figure just to make it easy.

The unit in question is cooling approximately 1200 sq ft, almost the exact same as the other 2 ton unit. The only difference is that the unit in question is cooling an area with vaulted ceilings, as the other 2 ton unit is cooling bedrooms/bathrooms with the normal 8 ft ceilings.

I even gone to the extent of measuring the temp/humidity on both stage 1 and stage 2 cooling, and both have elevated humidity levels at the supply's compared to the same settings on the other two units. Also, the air handler is set to factory defaults, just like the other two units. So, everything is identical on all 3 units, except this one blows air at twice the humidity???

I've checked the ducting for leaks, and there doesn't seem to be any. Also, all of the windows and sliders in the home were replaced 6 months ago with dual paned impact glass, and they are very well sealed. So, we've been able to rule out air leaks bringing in the humid air from outside.
I guess my question is this... If the unit is set up correctly (fan speed), why would it still be cooling the air, but not removing any humidity?

Do those vaulted ceilings have recessed lighting in them? If so, then you have holes in your ceiling that is letting hot humid air in. If ducts are run through unconditioned areas that are hot & humid, they could be bringing hot humid air into the home ( I know you stated they do not appear to have leaks). How long does the problem unit run? Just short run times?

An answer without a question is meaningless.
Information without understanding is useless.
You can lead a horse to water............http://www.mohomeenergyaudits.com

Yes, there is duct work in the attic, and all of it was replaced two weeks ago. And, the unit was doing the same thing with the old duct work. And I had it retested by the a/c company, and no leaks.

Second, I do not have any recessed/can lighting in the rooms.

And yes, the ceilings are all vaulted in this zone. 12-18 feet high...

And in response to the unit being over sized, I have the identical unit in the other end of the home and it is maintaining humidity levels in the low 40's%, as well as the larger unit in the main living areas. Just this unit, cooling a larger area than the other 2 ton unit is amiss... And not just by a little, it keeps the humidity at almost 25-30% higher???

So, Since I've had the a/c company out twice, and they told me nothing was wrong, I bought a humidity gauge and started some testing of my own. On the two units that seem to be able to maintain a comfortable level of humidity, these are readings: supply temp of 10-16 degrees below return air temp and humidity level at supply of 30-40%. On the unit I feel is malfunctioning, I am getting a supply temp of 10-16 degrees below return air temp, but the humidity levels are 75-90%.

Any suggestions on what is going on, and what I can tell my a/c company to look at? Or, is there anything else I can test to narrow the scope of the issue?

Well, I now see your last post & that changes what my response would have been.

We have to differentiate the variables between the higher humidity zone & the others.

OK, 1200-sf * 10' ceilings is 12,000-cu.ft., of air volume; +1500 -sf * 8' ceilings is also 12,000-cu.ft., of air volume.

What is the orientation of the home in relationship to the humid zone?

How much more exterior facing wall does it have, windows & doors, & as Tips asked: Do those vaulted ceilings have recessed lighting in them?

Hot attic air coming through any recessed lights or other areas or, into the Return-Air area can skyrocket the grains of moisture per lb of air at room temps - causing the SA humidity to be considerable higher.

By the way those other zones have unusually low SA humidity levels; which appears to mean there is a very low latent load in those two zones.

It would appear that there is a source of high humidity (high grains of moisture per lb of dry air) entering the problem zone.

Another factor could be a differential in the exact SA temp-drop between it & the other zones; also could be different runtime length & times on first stage verses 2nd stage between it & the other zones.

There will be real identifiable causes for the zone failure, they simply have to search effectively for those causes...

Well, I now see your last post & that changes what my response would have been. The duct leak would be in the Return-Air ducting not the supply; measure air temp just before entering blower, compare it to at RA grille air temps.

Last edited by udarrell; 08-07-2012 at 11:27 AM.
Reason: Now see UR last post; that changes what my response would have been

For the sake of easy explanations, the house is set up into an east wing, west wing, and a central wing. The two 2 ton units run the east & west wings respectively, while the 4 ton unit cools the central area in between. In regards to exterior facing windows, sliding glass doors, etc, the troubled unit has the least of all three units and has considerable shade exposure unlike the other two units.

Let me ask the question this way... If there isn't a huge leak of high humidity air entering the zone, is it possible there is something mechanically wrong with the air handler & coil or condensing unit? If so, what could it be? I want to be able to ask the right question the next time I call the tech out, so they just don't say "You need a dehumidifier". There is nothing structurally different in this end of the home, and the other two units keep a perfect humidity level. So, the excuse that I need a dehumidifier isn't one I'm willing to accept. If the other two units had similar issues, maybe I would consider it. Also, this isn't a new issue, as it has been going on for months....

The AOP forums/Ask Our Pro's forums, are restricted to only Pro members that have been vetted by the AOPC may post advise, questions or commentary in a thread created by someone else. You need to apply for your vetting/* if you wish to participate in threads in the AOP forums. Please apply to the AOPC today, thank you.

Well, hopefully tomorrow afternoon I will have some additional answers, as I scheduled another company to come out and look at the unit. Obviously the company that put the 3 units in hasn't been able to even diagnose an issue, so I'll try another and hopefully get someone that knows what they are doing... Does anyone have some suggestions of things I should make sure they check to ensure the unit is functioning properly?